The present invention relates to an olefin polymerization catalyst and a process for preparing a polyolefin by homopolymerizing or copolymerizing an olefin or olefins in the presence of the catalyst, the polyolefin having a relatively wide molecular weight distribution, good particle properties, a narrow composition distribution in copolymerization and a high molecular weight and being less sticky.
It is known in JP58-19309A to use a catalyst comprising a zirconium compound (typically a metallocene compound) and an aluminoxane in the preparation of a polyolefin, especially an ethylene homopolymer or an ethylene/.alpha.-olefin copolymer. This technique is advantageous in that ethylene copolymers can be produced in high yield, but disadvantageous in that the said copolymers are narrow in both molecular weight distribution and composition distribution and low in molecular weight.
If only the molecular weight of the resulting polymer is to be enhanced, this purpose can be attained to some extent by selecting a suitable transition metal compound in the metallocene as one component of the catalyst. For Example, with a view to improving the molecular weight of the resulting polymer, JP63-234005A proposes the use of a transition metal compound having a 2 , 3 and 4-substituted cyclopentadienyl group, and JP2-22307A proposes the use of a hafnium compound having a ligand bonded to at least two crosslinked, conjugated cycloalkadienyl groups.
However, the route and operation for preparing such catalyst components are complicated, and the use of hafnium as a transition metal is disadvantageous in that the yield (based on the catalyst used) of the resulting polymer is low. Besides, the conventional catalysts exemplified above are often soluble in the reaction system, so when used in slurry polymerization or vapor phase polymerization, there arises a problem such that the resulting polymer is extremely low in bulk density and inferior in particle properties.
Having made earnest studies for overcoming the above-mentioned problems of the prior art, the present inventors found out that a polyolefin serving a desired purpose, namely a polyolefin having a high molecular weight, a relatively wide molecular weight distribution, a narrow composition distribution and excellent particle properties could be obtained by using a novel catalyst system quite different in construction from conventional catalyst systems (JP5-132518A).
It is the object of the present invention to attain a further improvement, particularly to provide an olefin polymerization catalyst and a polyolefin preparing process using the catalyst which is extremely advantageous from the standpoint of productivity and which, in comparison with the prior art, uses a catalyst high in polymerization activity, especially in its activity at an early stage of polymerization, and affords a polyolefin superior in particle properties such as being high in bulk density, the polyolefin being substantially not deposited on a stirrer mounted in the reactor or on the inner wall of the reactor.